US Extended Forecast thru Dec 15

...Overview And Preferences...

Guidance Remains Consistent In Flattening The Large Scale Mean
Pattern Over The Conus With Time As The Influence Of Pacific Flow

Even With This Evolution Expect A Lingering Positively Tilted Mean Trough Into Next Sun... As Indicated By The Ensemble Means And Teleconnections Relative To A Core Of Negative Hgt Anomalies Over The Nrn Atlc And Positive Anomalies Over Europe.

Even With Reasonable Continuity/Agreement For The Evolution Of
Mean Flow There Is Still A Lot Of Spread Among Individual Model
Runs/Ensemble Members With Important Details Of Pac Flow
Approaching/Reaching The Conus And Also With Srn Canada/Nern Conus Flow.

An Even Blend Of The 06z Gefs/00z Ecmwf Ensemble Means Currently Provides The Best Starting Point For Resolving The Diffs
In The Guidance And Allows For Decent Continuity In Light Of The
Detail Uncertainties In The Fcst.

...Guidance Evaluation...

From Fairly Early In The Period There Is Considerable Spread And
Run To Run Variability With The Distribution Of Energy Within The
Leading Ern Pac Shrtwv That Should Be Approaching The West Coast
On Day 4 Thu... Which Ultimately Contributes To Significant Sfc
Diffs Over The Ern States By The Latter Half Of The Period.

Recent Ecmwf Runs Have Been Particularly Inconsistent With This
Leading Pac Energy While The Past Three 12-Hrly Gfs Runs Differ
With Details Over The West But Converge Somewhat Over The
Southeast By Day 7 Sun.

It Remains To Be Seen If The Shrtwv Reaching The Southeast Would Be As Strong As Seen In The Gfs Solns Given The Good Agreement/ Teleconnection Support Toward Somewhat Above Normal Hgts/Wswly Flow Aloft Over That Region.

Upstream The Mdls/Ensembles Advertise Potent Nern Pac Development In Assoc With Complex Nern Pac/Alaska/Sern Bering Sea Flow.

As Has Been The Case Recently Some Gfs Runs Are Trying To Pull This System Farther Swd Than The Established Consensus. The 06z Gfs And To A Lesser Extent 00z Gfs Exhibit This Issue.

In Addition..The 00z Gfs Becomes More Amplified With Trailing Flow By Day 7 Sun Leading To An Ern Trough/Wrn Ridge Pattern That Is Fairly Extreme Relative To The Full Ensemble Spread At That Time.

Across Canada And The Northeast... Diffs Aloft In The 00z Ecmwf
Lead To Its Run Having Much Higher Sfc Pressures Over/Near The
Grtlks Versus Other Guidance By Day 6 Sat. The 00z Ecmwf Mean
Shows Some Hint Of The Ecmwf Scenario With Higher Pressures Than The Gefs/Cmc Means. Recent Ecmwf Means Have Been More Suppressed With The Sfc Pattern Over This Area And Merit Half Weighting In Light Of The Cold Pattern In Place Over Cntrl-Ern Noam.

On The Other Hand Strength Of The Nern Pac System And Downstream Ridge Briefly Allows Potential For Somewhat Sharper Downstream Troughing Than Fcst By Ecmwf-Based Guidance.

...Sensible Weather Highlights...

Over The West Expect Initially Cool Temps To Trend Within A Few
Degs Of Normal As Pacific Flow Increases. One Area Of Mstr Should
Bring A Period Of Pcpn To Areas From The Nrn-Cntrl Coast Into The
Nrn Rockies Thu-Fri. There Is Still A Fair Amount Of Uncertainty
On Max Amts But They Should Be Focused Over The Pac Nw/Cascades. More Pcpn Is Likely To Reach Nwrn Areas Next Weekend.

Temps Over A Large Part Of The Cntrl-Ern States Should Remain
Below Normal... With The Fl Peninsula The Only Area Consistently
Above Normal Most Days.

The Most Extreme Anomalies Should Be Over The Nrn Plains/Upr Ms Vly Wed Into Thu Morning With Some Locations More Than 20 F Below Normal. Pcpn Should Be Fairly Lgt And Limited In Coverage During The First Half Of The Period... With Best Focus For Snow Downwind Of The Grtlks And Perhaps A Scattering Of Lgt Rnfl Over The Extreme Srn Tier.

The Fcst Becomes More Uncertain Fri-Sun In Light Of Guidance Spread With One Or More Pieces Of Energy Ejecting From The Swrn Conus/Nwrn Mexico.

Potential Exists For A Broad Area Of Mstr To Spread Across The Ern Half Of The Conus With The Large Scale Pattern Most Favorable For The Lwr Ms Vly/Tn Vly Region. The Nrn Part Of The Mstr Shield May Contain Some Wintry Pcpn.

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